Structural element for an aircraft, especially an aircraft door

ABSTRACT

Structural element for an aircraft, especially an aircraft door, in a single-piece, integral cast embodiment consisting of an aluminum or magnesium alloy.

[0001] The invention relates to a structural element for an aircraft,especially an aircraft door.

[0002] Structural elements for aircraft, especially aircraft doors,conventionally comprise a smooth outer skin, which is provided on theinner side with profiles, ribs and the like riveted or adhesively bondedto it, in order to achieve great stiffness and strength along with lowweight.

[0003] The production of structural elements of this type is complex,since the stiffening profiles and ribs are often produced from solidmaterial with a degree of machining of over 95%, and the riveting withclosely set rivets also requires considerable labor.

[0004] The machining of the profiles and ribs from solid material isnecessary in order for the final strength of the aluminum or magnesiumalloy used not to be impaired by the effect of heat. For the samereason, the connecting of the individual elements takes place byriveting or adhesive bonding and not by welding.

[0005] Although this conventional method of production allows structuralelements of minimized weight and with adequate strength to be produced,there is no assurance that stress or fatigue cracks will not form in theregion of highly loaded riveted connections or regions with high notchstresses.

[0006] The invention is based on the problem of providing structuralelements for aircraft, especially aircraft doors, which can be producedwith low weight and high strength at low cost and, on account of areduced risk of crack propagation, are easy to repair and service.

[0007] Taking this problem as a starting point, a structural element foran aircraft, especially an aircraft door, is proposed, producedaccording to the invention in a single-piece, integral cast embodimentof an aluminum or magnesium alloy.

[0008] Modern low-pressure sand casting processes in particular allowthe production of large single-piece, integral cast parts with uniform,thin wall thicknesses which do not require any, or considerably less,machining than the conventional structures assembled from individualelements. To improve the material properties, zircon sand may be usedfor the molds and cores, if appropriate in conjunction withsemipermanent molds.

[0009] The structural element may preferably comprise hollow,arched-bridge-like transverse members, longitudinal ribs connecting thefeet of the arches of the transverse members and sloping side facesconnecting and closing the end faces of the transverse members, and havereceptacles with gripping surfaces for straightening and machining. Inaddition, the structural element, especially in the embodiment as anaircraft door, may be provided with a peripheral flange, stiffened by aperipheral rib.

[0010] The structural element may in this case preferably be providedwith a smooth outer skin which is cast on as a single piece andintegrally, but alternatively may also be adhesively attached or rivetedon.

[0011] Furthermore, the structural element may be provided with ribswhich run approximately parallel to the rib at the flange and extendfrom the ends of one side face to the ends of the parallel other sideface at a distance from the outer feet of the arches of the outertransverse members and approximately perpendicular to the flange, andstiffening triangles running from the feet of the arches to the ribs maybe provided.

[0012] If the outer skin is cast on as a single piece and integrally,the flange may form an edge region of the outer skin.

[0013] In the case of the structural element according to the invention,accumulations of material and rough cross-sectional transitions can beavoided, with unavoidable cross-sectional widenings preferably beingarranged in the direction of feeding.

[0014] The arched-bridge-like formation of the hollow transverse membersallows adequately large openings for core removal to be provided;undercut core geometries are not required. The structural element has nodeep, thin slits, from which the core material is difficult to remove,and is provided with adequately large corner radii, facilitating shakingout.

[0015] The structural element may preferably be provided with a smoothouter skin, inwardly protruding, parallel longitudinal ribs, hollowelevated transverse members, reaching over the longitudinal ribs in anarched-bridge-like manner, side faces of the same height connecting andclosing the end faces of the transverse members, a peripheral flange,forming part of the outer face and stiffened by a peripheral, inwardlyprojecting rib, and receptacles with gripping surfaces forstraightening, adjustment and machining.

[0016] With the structural element according to the invention, simplemold parting lines with few cores, adequate drafts, adequate core crosssections for core production and handling and also appropriate coremountings can be achieved.

[0017] If the edges of the holes of the arched-bridge-like transversemembers are stabilized with beads, a slight distortion of the structuralelement occurs during a heat treatment following the casting operation.For a heat treatment which can be carried out without any problems, ribstructures with low internal stresses are used, rough cross-sectionaltransitions are avoided and notch stresses are reduced by means ofadequately large corner radii. The distortion of the structural elementremains low if air quenching is used during the heat treatment.

[0018] For cleaning the cast structural elements, all the mold and coreparting lines are easily accessible, no crossing cores are used,cleaning ribs are provided in the region of the parting joints to avoidthe surface of the workpiece being removed during cleaning, and as manycleaning regions as possible are arranged where machining issubsequently carried out.

[0019] Machining is performed in particular on the outer face of theouter skin and/or the inwardly directed surfaces of the transversemembers and/or the outer faces of the side faces connecting the endfaces of the transverse members and/or the inner face of the peripheralflange and/or the stiffening rib at the flange, on both sides, and/orthe gripping surfaces of the receptacles.

[0020] If the machining allowances for the outer skin are approximately6 mm for a machined wall thickness of approximately 1.5 to 2.7 mm and,for the remaining faces to be machined, approximately 3 mm for amachined wall thickness of approximately 3 to 4.2 mm, and the wallthickness of the cast, unmachined regions is 3 to 4.2 mm, a structuralelement can be produced in a single-piece, integral cast embodiment of aweight which is less than that of a conventionally produced structuralelement and the production and machining effort for which is likewisegreatly reduced.

[0021] The receptacles with the gripping surfaces are preferablyarranged on dimensionally stable regions, in particular at the fourcorners of a rectangle formed by the outer transverse members and theside faces connecting them.

[0022] The invention is explained in more detail below on the basis ofan exemplary embodiment represented in the drawing, in which:

[0023]FIG. 1 shows a perspective view of the outer face of an aircraftdoor according to the invention as a wire model,

[0024]FIG. 2 shows a perspective view of the unclad inner side of anaircraft door according to the invention as a wire model and

[0025]FIG. 3 shows an enlarged view of a corner region of the inner sideof the aircraft door according to the invention as a wire model.

[0026] The structural element according to the invention is, in the caseof the exemplary embodiment represented, an aircraft door in asingle-piece, integral cast embodiment consisting of an aluminum ormagnesium alloy and has a height of about 2400 mm and a width of about1300 mm.

[0027] The aircraft door is curved in a customary way with a radiuswhich corresponds to the curvature of the part of the fuselage where thedoor is fitted.

[0028] An outer skin 1, which has a completely smooth surface on theoutside, is connected to a load-bearing stiffening structure, whichcomprises longitudinal ribs 5, transverse ribs 6, elevated transversemembers 7 reaching over the longitudinal ribs 5 in an arched-bridge-likemanner, and side faces 2 connecting and closing the end faces of thetransverse members 7. A peripheral flange 3 forms part of the outer skin1 and is stiffened by a peripheral, inwardly projecting rib 4.

[0029] Provided on the narrow sides are ribs 17, which run approximatelyparallel to the rib 4 at the flange 3 and extend from the ends of oneside face 2 to the ends of the parallel other side face 2 at a distancefrom the outer feet 16 of the arches of the outer transverse members 7and approximately perpendicular to the flange 3.

[0030] In each case, one longitudinal rib 5 runs approximately centrallyunder the arch openings 12, while two further, parallel longitudinalribs 5 have a spacing corresponding to the width of the feet 16 of thearches and are connected to the latter. Arranged at the four corners inthe region of the ends of the upper and lower transverse members 7 andthe side faces 2 are receptacles 8 with gripping surfaces 9 for holdingduring heat treatment, straightening and machining. From the feet of thearches of the outer transverse members 7, stiffening triangles 13 extendto the ribs 17. Further stiffening triangles 14 are arranged along theside faces 2. Finally, stiffening triangles 15 are also provided in theregion of the feet 16 of the arches and the ribs 5 running there.Between the transverse members 7, the side faces 2 may be additionallysupported inwardly by means of reinforcing triangles, not represented,in order to avoid denting of the side faces 2 under flexural stress.

[0031] The edges of the arch openings 12 are stabilized by means ofbeads 11.

[0032] A rib structure which has low internal stresses and no roughcross-sectional transitions is achieved by largest possible transitionalradii between the individual elements. Adequately large corner radiiresult in only small notch stresses during the heat treatment aftercasting and in operation. The aircraft door with the structure accordingto the invention has particularly low distortion after the heattreatment, in particular when the aircraft door is quenched with airafter the heat treatment.

[0033] The structural element according to the invention produces simplemold parting lines, requires few cores and can be provided with adequatedrafts. The cross sections of the cores are adequately large, so thatthey are easy to produce and handle.

[0034] The structural element can be produced preferably by thelow-pressure sand casting process. The material properties are improvedby the use of zircon sand and/or a semipermanent mold.

[0035] With the structural element according to the invention,accumulations of material and rough cross-sectional transitions can beavoided; unavoidable cross-sectional widenings can be arranged in thedirection of feeding. The wall thicknesses can be made from a technicalcasting viewpoint in such a way that the wall thickness of the cast,unmachined regions is 3 to 4.2 mm, the wall thickness of the outer skinin the cast state is approximately 7.5 to 8.7 mm and that of theremaining faces to be machined approximately 6 to 7.2 mm.

[0036] After the machining, the wall thickness of the outer skin I isapproximately 1.5 to 2.7 mm, i.e. the machining allowance isapproximately 6 mm, while the wall thickness of the remaining faces tobe machined, i.e. of the flange 3, the rib 4, the surfaces of thetransverse members 10 before the machining are approximately 6 to 7.2 mmand after the machining are approximately 3 to 4.2 mm. Said machiningsurfaces are offset from the surfaces not to be machined of theunfinished part, and corner radii that are as large as possible areprovided everywhere.

[0037] To facilitate shaking out from the casting mold, adequately largearch openings 12 are provided in the transverse members 7, so that thehollow regions under the transverse members 7 allow easy core removal.There are no undercut core geometries nor any deep, thin slits fromwhich the molding sand is difficult to remove.

[0038] All mold and core parting lines are easily accessible andprovided with cleaning ribs, so that the risk of removing the surface ofthe material during cleaning is reduced.

[0039] Sprues, runners and feeders can be arranged in regions which aremachined, so that the effort involved in cleaning is low and robots canbe used for the cleaning.

[0040] The receiving points for straightening and machining, i.e. thereceptacles 8 with the gripping surfaces 9, are arranged ondimensionally stable regions of the structural element and areadequately dimensioned.

[0041] The structural element according to the invention has very goodstatic and dynamic strength properties along with a simple geometry,without requiring complex core technology. The wall thicknesses can beoptimized with regard to the stressing zones.

[0042] The exemplary embodiment represented shows an aircraft door castas a single piece and integrally, including the outer skin 1, but it isalso possible to produce a structural element as a cast part comprisingthe side faces 2, possibly the flange 3 with the rib 4, the longitudinalribs 5, the transverse ribs 6, the transverse members 7, the receptacles8 and the ribs 17, to machine surfaces of it, where required, andsubsequently to connect it to an outer skin 1 made of rolled sheet metalby riveting or adhesive bonding.

1. A structural element for an aircraft, especially an aircraft door, ina single-piece, integral cast embodiment consisting of an aluminum ormagnesium alloy.
 2. The structural element as claimed in claim 1,comprising hollow, arched-bridge-like transverse members (7),longitudinal ribs (5) connecting the feet (16) of the arches of thetransverse members (7) and sloping side faces (2) connecting and closingthe end faces of the transverse members (7), with receptacles (8) withgripping surfaces (9) for holding during heat treatment, straighteningand machining.
 3. The structural element as claimed in claim 2, with aperipheral flange (3), stiffened by an approximately perpendicular rib(4).
 4. The structural element as claimed in claim 3, with ribs (17)which run approximately parallel to the rib (4) at the flange (3) andextend from the ends of one side face (2) to the ends of the parallelother side face (2) at a distance from the outer feet (16) of the archesof the outer transverse members (7) and approximately perpendicular tothe flange (3).
 5. The structural element as claimed in claim 4, withstiffening triangles (13) running from the feet (16) of the arches tothe ribs (17).
 6. The structural element as claimed in one of claims 1to 5, with a smooth outer skin (1) cast on as a single piece andintegrally.
 7. The structural element as claimed in claim 6, in whichthe flange (3) forms an edge region of the outer skin (1).
 8. Thestructural element as claimed in one of claims 1 to 7, comprising asmooth outer skin (1), inwardly protruding, parallel longitudinal ribs(5), hollow elevated transverse members (7), reaching over thelongitudinal ribs (5) in an arched-bridge-like manner, side faces (2) ofthe same height connecting and closing the end faces of the transversemembers (7), a peripheral flange (3), forming part of the outer skin (1)and stiffened by a peripheral, inwardly projecting rib (4), andreceptacles (8) with gripping surfaces (9) for straightening andmachining.
 9. The structural element as claimed in one of claims 1 to 8,in which the edges of the holes of the arched-bridge-like transversemembers (7) are stabilized with beads (11).
 10. The structural elementas claimed in one of claims 1 to 9, in which the outer face of the outerskin (1) and/or the inwardly directed surfaces (10) of the transversemembers (7) and/or the outer faces of the side faces (2) connecting theend faces of the transverse members (7) and/or the peripheral flange (3)on its inner face and/or the stiffening rib (4) at the flange, on bothsides, and/or the gripping surfaces (9) of the receptacles (8) aremachined.
 11. The structural element as claimed in claim 10, in whichthe machining allowances for the outer skin (1) is approximately 6 mmfor a machined wall thickness of approximately 1.5 to 2.7 mm and, forthe remaining faces to be machined, approximately 3 mm for a machinedwall thickness of approximately 3 to 4.2 mm.
 12. The structural elementas claimed in one of claims 1 to 11, in which the wall thickness of thecast, unmachined regions is 3 to 4.2 mm.
 13. The structural element asclaimed in one of claims 1 to 12, in which the receptacles (8) with thegripping surfaces (9) are arranged on dimensionally stable regions. 14.The structural element as claimed in claim 13, in which the receptacleswith the gripping surfaces (9) are arranged in the four corners of arectangle formed by the outer transverse members (7) and the side faces(2) connecting them.
 15. The structural element as claimed in one ofclaims 1 to 14, with rib structures which have low internal stresses.16. The structural element as claimed in one of claims 1 to 15, withsmooth, rounded-off cross-sectional transitions.
 17. The structuralelement as claimed in one of claims 1 to 16, with large radii avoidingnotch stresses.
 18. The structural element as claimed in one of claims 1to 17, with air quenching during the heat treatment to reduce internalstresses in the component.